Three special ideal gas processes: one of, W or Q is 0

Transcription

1 Lecture 12 1st Law for isochoric, isothermal and adiabatic process Temperature change: specific heat Phase change: heat of transformation Calorimetry: calculating heat exchanges Specific heats of gases Heat transfer

2 E th Three special ideal gas processes: one of, W or Q is 0 fix volume by locking pin (piston massless, frictionless) change pressure by masses heat transferred thru bottom (piston/sides insulated) p g a s = p a t m + M g A (invalid when locked)

3 (i) Isochoric (W = 0 Q = E th ) lock pin; cool by ice; remove ice when desired p reached; remove masses till new p balanced; unlock... ii) Isothermal ( T = 0, E th =0 W = Q ) place over flame (gas expands); remove masses to match reduced p (pv = constant)... T =0 Q =0 (iii) Adiabatic ( ) Q =0 W = E th insulate; add masses (p increases, V decreases)... work done on gas increases temperature (same effect as Q) Q =0 T =0

4 Example A gas is compressed from 600 c.c. to 200 c.c. at a constant pressure of 400 kpa. At the same time, 100 J of heat energy is transferred out of the gas. What is the change in thermal energy of the gas during this process?

5 Thermal properties of matter (I) Joule: heat and work are energy transferred; change in thermal energy change in temperature or phase Temperature change and Specific Heat specific heat, c = energy to raise T of 1 kg by 1 K ( thermal inertia ) E th = Mc T (temperature change) E th = W + Q for solids/liquids, W =0 Molar specific heat, C: Q = nc T M(in g) Using n = = 1000(g/kg) M(in kg): M mol M (in g/mol) mol C(in J/mol/K) = M mol(in g/mol) c(in J/kg/K) 1000(g/kg)

6 Thermal properties of matter (II) Phase change and heat of transformation, L T same, heat transferred breaks bonds (instead of speeding up atoms) = 1 Mc L = heat energy for 1 kg to change phase L f, v : heat of fusion (solid/liquid) or vaporization (liquid/gas) L v >L f : bonds not completely broken during melting...

7 Calorimetry 2 systems interacting thermally, but isolated from others start at T 1 T 2, heat transferred till equilibrium T f (Q 1 is energy transferred to system 1: > 0 if energy enters...) Strategy for (> 2 systems) Q net = Q 1 + Q =0 Systems with temperature change: Q = Mc(T f T i ) Q>0 if T f >T i Systems with phase change: Q + ±ML f or v: for melting/freezing... (check: T f not higher/lower than all T i )

8 Example A copper block is removed from a 300 degree Celsius oven and dropped into 1.00 L of water at 20 degree Celsius. The water quickly reaches 25.5 degree Celsius and then remains at that temperature. What is the mass of the copper block? Specific heats of copper and water are 385 J / (kg K) and 4190 J / ( kg K), respectively.

9 same T Specific Heats of Gases, different Q since W different... Two versions of molar specific heat (if p or V not constant, use ) Q = W E th Relation between C P and C V E th depends only on T E th = Q + W no distinction between Q, W isochoric: ( E th ) A = W + Q = 0 + Q const vol = nc V T isobaric: ( E th ) B = W + Q = p V + Q const vol = nr T + nc P T (using ideal gas law: pv = nrt ) (E th ) A =( E th ) B 0 for isochoric, < 0 for isobaric expansion Q not same even if same: T Q = W + E th same

10 Heat depends on path E th = E th f E th i same (E th is state variable) W A + Q A = W B + Q B W B > W A (area under curve); W A, B < 0 Q B >Q A (Q, W are not state variables) Adiabatic Process (Q=0) e.g. gas in thermally insulated cylinder or rapid expansion/compression (no time for heat transfer via atomic-level collisions) (can be slow enough to be quasi-static) expansion lowers T... E th = W pv diagram (adiabat: steeper than isotherm) γ = C P C V = 1.67 monoatomic gas 1.40 diatomic gas Using ideal gas law p = nrt/v : TV γ 1 = constant

11 Heat Transfer: I (evaporation), II, III Conduction: T causes thermal energy transfer via object rate of heat transfer thermal conductivity (larger for good conductor: e.g. metals/diamond...with strong bonds) Convection: transfer of thermal energy by motion of fluid (``heat rises ): e.g. water on stove...(cf. conduction: atoms don t move from hot to cold side) winds; ocean currents (more rapid in water than air)

12 Heat Transfer: IV Radiation: electromagnetic waves (generated by electric charges in atoms) transfer energy from radiating to absorbing object: e.g. sun, lightbulb... Radiated power: emissivity (0 to 1) Objects also absorb: Stefan-Boltzmann constant: W/ ( m 2 K 4) Q net t = eσa ( T 4 T 4 0 ) environment e = 1: perfect absorber (no reflection) black body (also perfect emitter) climate/global warming (greenhouse effect): earth s atmosphere transparent to sunlight (visible)...cooler earth radiates infra-red (absorbed by )... CO 2